A symbiotic relationship between organisms. They can be helpful or harmful.
Mutualism
Mutualism is when both animals in a symbiotic relationship are helped.
One example of mutualism is a bee and a flower. The bee is helped by getting pollen. While the flower is helped because it is repopulating.
Commensalism
Another symbiotic relationship is commensalism. In commensalism one organism is helped and the other one is unaffected.
One example of example of commensalism is a squirrel and a tree. While the tree is unharmed, the squirrel get a home. Therefore because the tree isn't affected and the squirrel is help this is an example of commensalism.
Parasitism
Parasitism is a symbiotic relationship when one organism is helped and the other is harmed.
An example is fungus and a tree. when a fungus grows on a tree it takes all of its nutrients and the tree eventually dies. In this situation the host is the tree and the parasite is the fungus.
Competition
Competition is a symbiotic relationship between among living things for resource, such as food, space, shelter, mate, etc.
For example, an example of that would be two birds fighting over a piece of food.
Predator-Prey
A predator-prey relationship is the interaction between two organisms of unlike species in which one of them acts as predator that captures and feeds on the other organism that serves as the prey.
In the image below the bear is the predator and the prey is the fish.